Certain user equipments (UE) for use in telecommunications networks have requirements for low power consumption. Such UEs are typically not permanently attached to the network (as normally done by other devices such as, for example, mobile phone devices), as attachment to the network increases power consumption by the device.
An example of a device having low power consumption requirements is a Machine-Type-Communication (MTC) device. In short, MTC devices are a kind of UE/terminal arranged to communicate autonomously (i.e. without user intervention) over a telecommunications network with one or more servers (e.g. Application Servers (AS)) arranged to process, directly or indirectly, communications from/to the MTC devices. The 3GPP TS 23.682 discloses in chapter 4.2 the architecture reference model for a MTC device interfacing with a telecommunications network.
The telecommunications network through which the MTC device communicates can comprise either or both of a Packet Switch (PS) domain and a Circuit Switch (CS) domain. Each of the PS and CS domains comprises specialized servers. In the PS domain, the servers may include Serving General Packet Radio Service (GPRS) Support Nodes (SGSN) and Mobility Management Entities (MME). In the CS domain, the servers may include Mobile Switching Centers (MSC) and Visitors Location Register (VLR). In addition, servers may be common to both the PS and CS domains such as a Home Subscriber Server (HSS). 3GPP TS 23.682 specifies in chapter 4.5.4 a mechanism to optimize network signaling for UEs requiring lower power consumption and that expect infrequent mobile originating and terminating services and that can accept a corresponding latency in mobile terminating communication. This mechanism is implemented by using a new UE mode called Power Saving Mode (PSM). A UE using PSM is available for terminating services only for the period of time referred to as Active Time. The Active Time is a period of time specified after a mobile originated event like data transfer or signaling. Outside of the Active Time, the UE is not reachable for terminating communications till its next Active Time.
When a UE wants to use PSM it specifies an Active Time value during all its registrations (either: initial or re-registrations). Namely the UE inserts an Active Time information element on different registration procedures; such as: Attach, Tracking Area Update (TAU) and/or Routing Area Update (RAU) procedures from the UE to the corresponding serving node (e.g. SGSN/MME—SGSN for 2G/3G and MME for Long Term Evolution (LTE)). If the network supports PSM and accepts that the UE uses PSM, the network confirms usage of PSM by allocating an Active Time value to the UE.
If the network has allocated an Active Time value, the Mobile Station (MS) starts an Active Timer and the SGSN starts a Mobile Reachable timer with the value of Active Time. When the Active timer expires, the UE deactivates Access Stratum functions and enters PSM for the UE. In PSM, due to deactivation of Access Stratum functions, the UE stops all idle mode procedures, but continues to run any Non Access Stratum (NAS) timers that may apply, e.g. periodic RAU timer.
When the mobile reachable timer expires, the SGSN/MME knows that the UE has entered PSM and is not available for paging so it stops sending paging messages to the UE. This means that terminating services stop at SGSN/MME which rejects them and does not even attempt to page the UE. For the case of Short Messaging Service (SMS), as the UE is not paged, the SGSN/MME rejects the SMS but the SMS Service Centre (SC) can store it till the UE enters the Active Time again. For the case of IMS services the SGSN/MME rejects the communication so the service is lost.
PSM has no support in the CS domain on the network side. 3GPP TS 23.682 states that PSM should only be used by UEs using the PS domain, SMS and mobile originated IMS or CS services. A UE that uses mobile terminated IMS or CS services other than SMS should not use PSM as neither IMS nor the CS domain provide support for mobile terminated CS voice or IMS services to UEs that are in PSM.
3GPP TS 23.887 in chapter 7.1.3.3.1 describes a mechanism where a Service Capability Server (SCS) or other entity may decide to subscribe in an HSS to receive notification when a particular MTC device becomes reachable, e.g. when an Active Time starts. If the terminating CS or IMS service fails because the device was in PSM, the SCS should not retry the service but wait till it receives the notification of reachability from HSS.
Currently PSM is only supported by nodes of the PS domain, e.g. the MME and/or SGSN, but not by nodes of the CS domain or IMS network. For example, 3GPP TS 23.682 in chapter 4.5.4 recommends that a UE that uses terminating IMS services or CS terminating services other than SMS should not use PSM.
In the case of terminating IMS services, as the network does not support PSM and the HSS is not aware of the PSM status of a UE, the IMS service request will reach the serving node (SGSN/MME). If the UE is in PSM the SGSN/MME does not page the UE as the UE is not reachable and the service request is rejected. IMS/HSS resources have been wasted.
In the case of Terminating CS services, as the MSC/VLR does not support PSM functionality, the terminating service request is handled and the UE will be paged even when it is in PSM mode so HSS, MSC/VLR and radio resources are wasted.
The solution mentioned above where the SCS registers in the HSS for notifications of when a UE, such as an MTC, becomes reachable, would not prevent the SCS, or other applications, from sending terminating traffic at any time without waiting for the HSS notification and thereby wasting network resources.